Wireless radio frequency (RF) receivers are designed to have a bandwidth of operation that is very close to the bandwidth of the signal of interest. Due to the small margin between the operational bandwidth of the receiver and the bandwidth of the signal of interest, such receivers can receive even low-powered signals while maintaining a relatively high signal-to-noise ratio (SNR). However, the small bandwidth margin also makes such receivers susceptible to tuning errors. Even a small tuning error can result in the signal of interest falling partly or completely outside the receiver's pass band, severely degrading receiver performance.
Tuning errors are often caused by inaccuracy and drift in the receiver's reference clock. For example, less expensive crystal oscillators tend to be less accurate and exhibit greater temperature dependence than more expensive crystal oscillators. Consequently, one way to avoid receiver tuning error is to use a highly accurate, temperature-invariant crystal reference. However, such components are too expensive to be used in many consumer products (e.g., electronic pagers, wireless modems, cell telephones, etc.) and it is preferable to design receivers to use lower cost, less accurate reference clocks.
One technique for tolerating a less accurate reference clock while maintaining a relatively high SNR is to use automatic frequency control (AFC) to adjust the tuning frequency of the receiver. Because the passband of the receiver is typically centered around the tuning frequency, adjusting the tuning frequency up and down effectively shifts the passband of the receiver up and down. Using AFC, the operational bandwidth of the receiver is effectively increased by the frequency range over which the tuning frequency can be adjusted, yet a narrow passband is maintained.
AFC is typically implemented by using negative feedback to control the output of a voltage-controlled crystal oscillator (VCXO). The VCXO outputs a tuning signal having a frequency that is increased and decreased according to the level of a control signal. The control signal is generated based on the difference between the frequency of the tuning signal and the frequency of the signal of interest (SOI). When the tuning signal frequency falls below the SOI frequency, a positive control signal is produced to increase the tuning signal frequency. Similarly, when the tuning signal frequency rises above the SOI frequency, a negative control signal is produced to decrease the frequency of the tuning signal.
A significant disadvantage of the above-described AFC scheme is that its frequency range is limited due to the fact that the receiver must determine the SOI frequency in order to properly control the VCXO. If the SOI falls outside the initial passband of the receiver, the SOI frequency cannot be determined and a reliable feedback signal cannot be supplied to the VCXO to adjust the tuning signal frequency. Consequently, the above-described AFC technique can typically increase the operational bandwidth of a receiver only by an amount equal to the SOI bandwidth.